A vertical cavity surface emitting laser (VCSEL) emits light in a beam vertically from it surface. Light emitted from a VCSEL is typically focused by a transfer lens into an optical fiber and used for the transmission of data. Transmission technology such as Gigabit Ethernet technology utilizes VCSELs and multimode fiber optic cabling.
The ever-increasing data rate across multimode fiber optic systems requires more sophisticated coupling optics for the transmitter module to satisfy the required bit-error rate.
There are two important considerations in the design of a transfer lens: 1) reflection management, and 2) creation of a favorable launch condition. The first design consideration of reflection management seeks to minimize the amount of light that is reflected back from the surface of the optical fiber (referred to as “back reflections” or feedback) and directed to the light source (e.g., the laser). When reflections are not managed properly, the back reflections can cause stability problems for the laser source. Specifically, if these back reflections are not controlled or reduced, the laser can become de-stabilized and may operate with a noisy output signal. For example, when too much power is coupled back into the laser from the reflection from the end of the optical fiber, instabilities occur in the laser, and the output power oscillates up and down, thereby causing extra and damaging amounts of jitter as the received signal pulses. In other words, instability in the laser causes erroneous data signals.
Furthermore, the increased noise in the laser that is induced by the coupling lens can lead to a power penalty in the optical budget of the data link as high as 2.5 dB. It is evident that the increased power penalty due to the back reflections represents a significant fraction of the total link power budget which, for a 2.5 Gbit/sect data rate, is on the order of about 8 dB. This adverse effect of back reflections or feedback becomes more pronounced and significant for higher data rate systems. For example, the power budget for a 10 Gbit/sec link becomes even more taxed than the 2.5 Gbit/sec link.
Second, it is important that the transfer lens design provide a favorable launch condition at the fiber interface in order to maximize the bandwidth-distance product of the system. For example, for a standard 50 micron graded-index fiber, a 2.5 Gbit/sec link requires a bandwidth-distance product of 500 MHz*km. Similarly, for a 10 Gbit/sec link, the fiber needs to support a product of 2.2 Ghz*km.
A favorable launch condition should increase bandwidth of the system and is robust to lateral offsets (i.e., misalignment between the laser and the fiber). One approach to improve favorable launch condition is to avoid launching the light along the very center of the fiber. A reason for avoiding the center of the fiber is that many fibers have defects along the center of the fiber due to manufacturing limitations. Furthermore, tolerance for lateral offsets is desirable to compensate for any misalignment between the laser and the fiber. Otherwise, misalignment in the system (e.g., misalignment between the optical fiber and transfer lens or the misalignment between the transfer lens and the laser) may cause the light from the laser to miss the optical fiber.
Unfortunately, the prior art transfer lens designs have shortcomings in either addressing back reflections or providing favorable launch conditions. These shortcomings and disadvantages stem primarily from constraints and difficulties in lens fabrication.
Diffractive Vortex Lens for Mode-Matching Graded Index Fiber
There have been some attempts to use diffractive elements as coupling optics to launch light into graded index fiber. One such study is reported by E. G. Johnson, J. Stack, C. Koehler, and T. Suleski in the Diffractive Optics and Micro-Optics, Optical Society of America (OSA) Technical Digest, pp. 205-207, Washington, D.C., 2000, in an article entitled, “Diffractive Vortex Lens for Mode-Matching Graded Index Fiber.” This publication describes an approach that utilizes a diffractive element to match the phase of the launched light into specific modes of the graded index fiber.
While these prior art approaches provide tolerable results for ideal point light sources (i.e., light that has a simple distribution and is perfectly coherent), these approaches do not adequately address applications that employ light sources with more complex light distributions (e.g., a multi-mode laser). In these specific real-world applications, the prior transfer lens suffer from more destabilizing feedback due to poor management of back reflections, unfavorable launch conditions stemming from larger amounts of on-axis energy, or both.
Based on the foregoing, there remains a need for a transfer lens that simultaneously reduces back reflection and provides favorable launch conditions and that overcomes the disadvantages set forth previously.